Semiconductors and the like hereafter referred to as objects to be cleaned are placed inside a pressurized cleaning vessel in which supercritical fluid is fed for cleaning of the said objects to be cleaned by contacting with supercritical fluid and dissolution of impurities adhered to the objects. The supercritical fluid is circulated. When eliminating the supercritical fluid in which these contaminatns dissolved from the pressurized cleaning vessel by a suitable means, the objects which were precisely cleaned are obtained. A troublesome process for drying the objects required in so-called wet cleaning methods is unnecessary since the supercritical fluid is completely vaporized by a decompressing operation.
It is possible to reuse the supercritical fluid because such supercritical fluids with the above-mentioned contaminants are transferred from the cleaning vessel into a separation tank (or a recovery tank) and then the solvent is separated from the contaminants by mechanism stated whithin.
As shown in the FIG. 7 concerning a schematic plot of the states of the pure substance and Its supercritical state, this is the latest technique for cleaning semiconductor substrates by using the following two characteristics of the supercritical fluid in its high density fluid state in which condition of pressure and temperature at the near critical region is P&gt;P. (crirical pressure) and T&gt;T. (critical temperature).
Characteristic 1: It is possible to easily obtain a great change in density with a slight change of pressure. It is also possible to obtain a great difference or solubility because the solubility of substance is in general, proportional to the density thereof.
Characteristic 2: The density of a supercritical fluid has low viscosity and high diffusivity although it is similar to liquid. Therefore, such a characteristic is advantageous when a 'substance moves.
The priciple shows that firstly, the low viscosity of the supercritical fluid can easily enter into narrow pores and crevices. On the contrary, for the case of high viscosity thereof, the fluid has high solubility for contaminants adhered or penetrated into a substrate, especially organics substances. Secondly, the above-mentioned cleaning operation is achieved with the supercritical fluid containing the contaminating substance, and the contaminants can be divided between solid and liquid states because solubility of the said contaminants is weakened by decreasing the density thereof. Accordingly, in this invention, it is possible to recover these components by a decompression operation in a separation vessel.
After the contaminants are separated from the solution, the purified supercritical fluid used in the pressurized cleaning vessel can be stored and recycled resulting in a closed recycle system.
Applicable gases in this invention are carbon dioxide, nitrous oxide, ethane, propane and the like. In particular, carbon dioxide is highly suitable to handle since it is nonflammable, nontoxic and inexpensive, and has a critical temperature of 31.1.degree. C. and a critical pressure of 72.8 atmospheres.
In such a recycle system mentioned above, a general fluid tranfer method can be accomplished with a high pressure generator such as compressors, pumps, etc. to make and keep the supercritical state. However, such equipment as cleaning equipments are very costly since these equipments need the high ability to generate high pressure, and further these methods are known to generate fine particles that are detrimental to the cleaning process.
Even though it is possible to depend only upon the use of solvents for cleaning, for a case when this is not effective, it is available to use forced means, for example agitation and/or ultrasonic energy for promoting cavitation in order to impart cleaning action. The various contaminants to be difficult to dissolve can be effectively dissolved with addition of an entrainer.
The above-mentioned characteristics of the supercritical fluid are to include near-critical fluids. As shown in the pressure-temperature diagram of FIG. 7, a near-critical fluid denotes that a fluid exists in the neighbouring region of the critical point and the fluid exists as a compressed liquid and a compressed gas. Fluids in this region and supercritical fluids behave distinctly. What exists in the near critical region or the supercritical region of the neighbouring region point above the critical temperature is given the name of a high density liquefied gas.
As an example, there is a simple cleaning method to reduce cost disclosed in the Japanese Patent Publication No. 7-171527 (1995), which teaches a means as to a high density liquefied gas used as solvent in the critical region and supercritical fluid but requires high pressure major equipment components.
The features in said Japanese Patent Publication are a means of temperature control provided in a cleaning vessel and the use of ultrasonic energy for cavitation. Namely, what was described in said Publication is only one means to improve cleaning efficiency with temperature control using a high density liquefied gas in a cleaning vessel with cavitation produced from ultrasonic energy.
However, this method teaching in said Publication is still insufficient for reducing cost since a pump and the like are employed as the power to transfer a high density liquefied gas in a process for removing contaminants and recoveriing solvent in a closed recycle system. Moreover, particles and other contaminants. either as metals, inorganics, degraded seals or gaskets or as lubricants, oils or other undesirable fluids, are introduced into the cleaning system through use of these high pressure major equipment components. Furthermore, in this method mentioned above, it is not sufficient to completely remove contaminants by cavitation with strong agitation for supplementing reduction of the cleaning advantage of high permeability in a supercritical state because only the partially projected contaminants are removed. Although cleaning in this method is carried out by immersing the objects to be cleaned in liquid which is indispensable for cavitation, contaminants in the liquid re-adhere onto the objects due to many solid contaminants being In the liquid. Thus, such a method in prior art has an inconvenient factor because there is a re-cleaning step as another process for achieving complete cleaning. Further, for the case of using a high density liquefied gas including CO.sub.2 as solvent having the critical temperature in the vicinity of room temperature, it is hardly expected that contaminants adhered on inorganic compounds, polar substances and others having remarkable characteristics of being difficult to dissolve are perfectly removed even though entrainers were added for increasing the cleaning effect. Therefore, at the present time, there is great anticipation among these in the trade of how such a difficult problem can be solved simply and improved for industrial applications.
In view of the above-mentioned contemporary circumstances that are noted to be difficult to improve, one objective in this invention is to provide a new method using a high density liquefied gas for achieving the cost reduction, remarkably powerful cleaning, an effective cleaning means for solving a difficult problem as to re-adhering and especially. a complete cleaning of contaminants consisting of inorganic compounds, polar substances, etc., even if the solvent has the critical temperature in the vicinity of room temperature.